Static RAM or SRAM, which is a kind of semiconductor memory, is able to preserve data therein only as long as power is supplied to a memory, unlike DRAM (dynamic RAM), in which data is periodically renewed. Since SRAM is a kind of random-access memory, the time required to input and output data is constant regardless of the addresses at which data is written and read. SRAM is a memory element that is completely different from SDRAM, which is a kind of DRAM, and thus these should be distinguished from each other. In SRAM, each bit is stored in two pairs of inverters comprising four transistors. Two pairs of inverters keep the values of 0 and 1 stable and two access transistors perform reading and writing functions. Thus, six transistors are typically required to store one bit.
Gordon Moore, the co-founder of Intel, elucidated Moore's Law in 1965, which states that the degree of integration of semiconductor transistors doubles every 18 months. In the decades that followed, the degree of integration of semiconductors has increased in accordance with Moore's Law, but recently, Moore's Law has become inapplicable due to the physical limitations of atomic-scale processing. Therefore, progressively intensive research is ongoing into increasing the degree of integration of semiconductors by vertically stacking semiconductors, rather than merely increasing the degree of integration in a plane.
Meanwhile, a thin-film transistor (TFT) is provided in the form of a thin film, which is a kind of a field-effect transistor (FET). Basically, it is a three-terminal element (no back gate B is present), and is mainly applied to liquid crystal displays.
Also, an organic field-effect transistor (OFET) is a field-effect transistor in which an organic semiconductor technique is used in the transistor channel. The organic field-effect transistor may be manufactured using a vacuum evaporation process on a small molecule or a solution-casting process on a polymer. The organic field-effect transistor has been developed to manufacture large-area electronic products at low cost, and the organic field-effect transistor is manufactured with various device geometries.
Recently developed flexible thin-film transistors based on soft materials have significantly lower electron mobility than silicon semiconductors. For this reason, in order to achieve similar levels of performance to existing silicon-based semiconductor devices, the channels of individual semiconductor devices must be hundreds to tens of thousands of times larger. Moreover, in order to manufacture a memory element, semiconductors of two different types, namely a p-type and an n-type, have to be formed on a single surface and connected to each other, and thus a p-type semiconductor material, an n-type semiconductor material, electrodes and insulating materials suitable for respective types are patterned, which thus complicates processing.
As described above, the related art is problematic in that it is difficult to apply soft materials and processes to memory elements because the number of elements integrated per unit area is very small and the processes are complicated.